A border patrol agent slain in the line of duty last week was likely killed in an accidental early morning gunfire exchange involving another border patrol agent who was also injured, preliminary FBI information indicates. The news comes despite early speculation that two individuals connected to a drug smuggling operation were responsible.

Nicholas Ivie, 30, a six-year border patrol veteran, was patrolling an area of Southern Arizona near Naco at about 1:30 a.m. last Tuesday and went to investigate a tripped border sensor. Officials believe he may have come across another group of agents who were doing the same. The rugged, hilly area may have provided enough nighttime cover for the agents to believe they had come across smugglers, with which the area has been long been plagued.

George McCubbin, president of the National Border Patrol Council, a union representing border patrol agents, told the Associated Press that Ivie may have fired several shots, while the other agents returned fire, killing him. “I don’t know what it was he saw or heard that triggered this whole event,” McCubbin said. “Unfortunately it resulted in his death and another agent injured,” said McCubbin. The Arizona Republic reported that the each group of agents knew the other was investigating the sensor, but lost radio contact as they approached.

In statements, both the U.S. Border Patrol and the FBI both acknolwedge their belief that Ivie was killed by friendly fire, although officials had thought earlier that he died at the hands of drug runners. On Thursday, two men were reportedly arrested in connection with Ivie’s death; now, based on the official statements, it seems unlikely that there was any link between those arrests and the Border Patrol incident.

Ivie was laid to rest on Monday in a service at a Sierra Vista, Ariz., Mormon church. The investigation into the incident is continuing, officials say, but a review of protocols is expected to be put in place to avoid future tragedies.

“They had a couple of people in custody but other than being in the area, there was no evidence putting them there at the scene,” McCubbin said. “They could have been guides, they could have been scouts, and those type of folks typically go back and forth all the time anyway.”